Bioactive nucleic acid pharmaceuticals including small interfering RNA (siRNA) and antisense nucleotides are expected to be next-generation drugs for treating, e.g., cancers and viral diseases. However, practical application of nucleic acids has been limited at present, since such nucleic acids are essentially unstable in vivo and have low bioavailability. In order to apply nucleic acid pharmaceuticals to treatment of a wider range of diseases, an efficient and safe nucleic acid delivery system that enables systemic administration is required (see Non-Patent Documents 1 and 2 below). Viral vectors are known to be able to deliver nucleic acids to target sites at high efficiency, but their clinical uses are limited due to, e.g., possible immunogenicity and carcinogenicity (see Non-Patent Documents 3 and 4 below). Therefore, nonviral vectors composed of cationic polymers and/or cationic lipids are attracting attention (see Non-patent documents 5 and 6 below).
Although a cationic polymer forms a complex with a nucleic acid via electrostatic interaction, such complexes are unsatisfactory in particle size control and stability in blood for the purpose of efficient delivery to targets. To address this problem, a method has been proposed which uses a cationic polymer bound to a water-soluble polymer having high biological affinity such as polyethylene glycol (PEG) (see Patent Documents 1, 2, 3, 7, and 8 below). The resultant complexes become core-shell particles having a hydrophilic shell of PEG and a core bound to a nucleic acid, thereby enabling control of particle diameter. However, it has been reported that these complexes are quickly eliminated from blood immediately after intravenous administration (see Non-Patent Documents 9 and 10 below), so that further stabilization is desired.
In brief, Patent Document 1 describes a block copolymer having a hydrophilic segment and a charged segment, as a carrier for targeted delivery of DNA, which is a charged molecule. However, a nucleic acid complex formed from this copolymer may exhibit insufficient stability in blood, as described above. Patent Document 2 is intended to address such a problem, i.e., to specifically stabilize a polymeric micelle formed of a plurality of such block copolymer molecules in an aqueous medium, by introducing mercapto groups into the charged segment of the block copolymer so that disulfide bonds is formed in the polymer molecules. Patent Document 3 provides a graft polymer in which a PEG chain (hydrophilic group) and a palmitoyl group (hydrophobic group) are bound to poly-L-lysine via ε-amino groups of the side chains.
On the other hand, another method for stabilizing a polymer micelle has been proposed, which includes preparing a cationic polymer, one end of which is bound to a PEG chain (e.g., PEGylated poly(N-methyldiethanamine sebacate), and grafting cholesterol (hydrophobic group) as a side chain to the PEGylated cationic polymer (see Non-Patent Document 11 below). Although this reference does not report on the evaluation of blood retention, it is believed that introduction of a hydrophobic group results in stabilization of the particles to a certain extent. However, preparation of this polymer requires harsh conditions (120° C., 24 hours), which may cause decomposition of the polymer, and thus it is thought to be difficult to stably control the molecular weight of the polymer and the introduction rates of PEG and cholesterol. This method also requires preparing a complex at an acidic condition of pH 4.6, which makes DNA unstable (see Non-Patent Document 12 below). These problems prevent the use of this polymer in pharmaceutical formulations. Furthermore, this reference teaches that PEG having a molecular weight of 2000 at maximum should be used at a concentration of from 1.8% to 8.2% with respect to the polymer, but such conditions will prevent sufficient formation of a hydrophilic shell. In fact, the particle diameter of the complex with DNA is as large as 200 nm or greater, and the zeta potential of the particle exhibits a negative or positive value reflecting the properties of the core. Such properties will be an obstacle to achieving high blood retention (see Non-Patent Documents 6 and 13 to 17 below, which also include other information regarding the field of the invention).